Flexographic printing involves inking a raised image which then comes in contact with the print substrate, for instance paper or plastic, and the transfer of ink from the raised image onto the print substrate. The plate is made of a rubbery material which has a somewhat pliant nature, the extent of which depends on the smoothness and fragility of the substrate. In contrast to other print processes such as offset lithography and gravure where high pressure is used during ink transfer, it is generally desirable to have a minimum of pressure between the raised inked image on the plate and the substrate. Too little pressure and no ink transfer or very uneven ink transfer will occur. Too much pressure and the pliant surface of the plate will be squashed into the substrate causing blurring of the image edges resulting in poor print quality.
Because of the requirement to work at minimal pressure for optimum quality, the distance between the plate surface and the substrate must be the same over the entire surface. This may depend on the uniformity of the press cylinder on which the plate is mounted and on the plate thickness uniformity. In the book Flexography Principles and Practices (Fourth Edition, page 109) accuracies of plus or minus 0.0005 inches are needed for the printing plates.
For some years the dominant type of flexographic plates has been based on mixtures of elastomeric material, photosensitive monomers and photoinitiators. Such plates have been termed polymer plates and as such they are supplied to the customer as solid light-sensitive plate material. These plates are generally made to the above-mentioned tolerance. For instance, U.S. Pat. No. 4,272,608 (Proskow), describing the manufacture of such plates, states that they can be made by solvent casting or by extruding, calendaring, or pressing at an elevated temperature. A further development in plate technology was in the introduction of LAMS plates-laser ablated masks. A black layer is coated on the photopolymer plate and then ablated away in areas that will correspond to the print image. The plate is exposed to UV light and developed. However accurately the plate is made, there is some distortion due to solvent development. This problem was discussed in U.S. Pat. No. 5,252,432 (Bach et al.). Using suitable choice of photopolymers and developer liquids they were able to achieve a thickness tolerance after development of +/− less than 15 microns.
An alternative way of preparing flexographic plates and sleeves is by engraving with a laser by ablation. Such a process does not require solvent development and therefore changes of thickness from such a cause are eliminated. For sleeves, the flexographic rubber has to be applied to a sleeve shell. U.S. Pat. No. 4,144,812 (Julian) describes such a process and grinding to obtain uniformity of thickness required. Such a method of grinding, however, was discussed in U.S. Pat. No. 5,798,202 (Cushner) as being time consuming and labour intensive.
Flexographic printing has increased applications in high print quality products which had previously been dominated by gravure and litho printing. For instance, plate-making is much easier and quicker than gravure and the use of inks where the carrying media is evaporated for drying makes it more applicable to printing on polymer than offset litho. The roll-to-roll flexographic machine is simpler than any roll-to-roll offset press which would be needed to print for instance flexible packaging.
For higher quality flexographic printing the plate thickness uniformity becomes an even more important issue. An additional part of obtaining high quality flexo printing is to use a soft under-cushion. During printing this cushion provided the give which would otherwise be provided by the plate image surface which would then slightly distort. However, generally the cushion has an even wider thickness tolerance than the plate itself.
A challenge of all mass production is quality control. For instance, in the case of flexographic plate precursor sheets, mass production is done in a continuous manner and control of thickness must be monitored and adjusted to always be within the specification. There is always some possibility, however, that plate precursor material that will be outside the thickness specification, will escape notice, and reach the customer. Such defects may be visually undetectable and would only be seen once the plate is imaged during the printing process. While the manufacturer may accept responsibility for plate defects and replace any plates, they would be unlikely to recompense the customer for the cost of time, materials, and inconvenience involved. The only way the manufacturer could ensure that this does not happen would be to check each plate precursor in a way that would not be economically viable.
The present invention solves a recognised need to ensure that the customer can optimise plate quality so that they are not wasting time and money in imaging and printing inferior plates.